This invention relates to apparatus for reproducing four separate channels of information after recording or transmission on a medium having only two tracks, and presenting it on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. More particularly, the present invention is concerned with the modification of the four signals obtained from a preceding quadraphonic matrix decoder not forming part of the invention in such a way as to enhance the directional content of said signals before presenting them to the loudspeakers.
Previous methods of enhancing the directionality of such signals have been incorporated as part of the matrix decoder and designed and intended for use with one particular quadraphonic system such as the SQ system of Columbia Broadcasting System, Inc. and the QS system of Sansui Electric Co., Ltd. The present invention differs from these in that it accepts the four outputs from a simple matrix decoder operating on any such system, and does not therefore include a 2-4 matrix decoder as such. Although there are differences in the mathematical formulation of the various systems, it is possible for the enhancement system to operate effectively with any system and to be switched from one system to another with only minor changes in the detailed circuitry. The enhancement system operates on the same principles whatever quadraphonic matrix is used.
In previous types of logic-directed decoders or similar systems, the approach has either been piecemeal resulting in only modest improvements, or has been unable to carry out the function of separating the signals whilst at the same time maintaining absolute constancy of the total power output due to any source being reproduced. This is important because the psychoacoustic effect on which such systems depend is sensitive to variation of the total power. By adopting a holistic approach to the problem, a new mathematical basis has been formulated for the enhancement system, and the method thus devised has been implemented in a novel way.
Previous systems of this kind have employed devices such as variable gain amplifiers, photoconductive cells or field-effect transistors to accomplish the limited objectives. In some cases, little effort has been made to reduce unwanted effects such as harmonic distortion. One of the features of the present invention is that it can be embodied in a way which maintains very low harmonic distortion, and is therefore compatible with the best high fidelity sound reproducers.
The invention also embodies improved methods of detecting the direction of the predominant signals and for imposing suitable limiting and attack-decay characteristics on the control signals thus obtained.
While the invention is primarily designed for use in a quadraphonic sound system, the invention has broader utility in that it can be used to enhance the directionality of any such signals having directional information. The enhanced signals could then be applied to an appropriate device other than a loudspeaker. For instance, the method of this invention could be applied to a known communication system in which five separate transmission channels are employed to carry ten simultaneous messages, each message being transmitted on a different pair of the five channels. Further, the method of this invention is not limited to any specific number of channels.